A CMOS imager includes a focal plane array of pixel cells, each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. Typical CMOS imager pixel cells have either a three transistor (3T) or four transistor (4T) design. The 4T design is preferred over the 3T because it reduces the number of “hot” pixels in an array (those that experience increased dark current), and it diminishes the kTC noise that 3T designs may experience with the readout signals.
In a CMOS imager, the active elements of a pixel cell, for example a four transistor pixel, perform the necessary functions of (1) photon to charge conversion; (2) transfer of charge to a floating diffusion region; (3) resetting the floating diffusion region to a known state before the transfer of charge to it; (4) selection of a pixel cell for readout; and (5) output and amplification of signals representing a reset voltage and a pixel signal voltage, the latter based on the photo converted charges. The charge at the floating diffusion region is converted to a pixel output voltage by a source follower output transistor.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630; 6,376,868; 6,310,366; 6,326,652; 6,204,524; and 6,333,205, all assigned to Micron Technology Inc. The disclosures of each of the foregoing are hereby incorporated by reference herein in their entirety.
A conventional CMOS APS (active pixel sensor) four-transistor (4T) pixel cell 10 is illustrated in FIGS. 1A and 1B. FIG. 1A is a top-down view of the cell 10; FIG. 1B is a cross-sectional view of the cell 10 of FIG. 1A, taken along line A-A′. The illustrated cell 10 includes a pinned photodiode 13 as a photosensor. Alternatively, the CMOS cell 10 may include a photogate, photoconductor or other photon-to-charge converting device, in lieu of the pinned photodiode 13, as the initial accumulating area for photo-generated charge. The photodiode 13 includes a p+ surface accumulation region 5 and an underlying n-type accumulation region 14 formed in a p-type semiconductor substrate layer 2.
The pixel cell 10 of FIGS. 1A and 1B has a transfer gate 7 for transferring photocharges generated in the n-type accumulation region 14 to a floating diffusion region 3 (i.e., storage region). The floating diffusion region 3 is further connected to a gate 27 of a source follower transistor. The source follower transistor provides an output signal to a row select access transistor having a gate 37 for selectively gating the output signal to an output terminal (not shown). A reset transistor having a gate 17 resets the floating diffusion region 3 to a specified charge level before each charge transfer from the n-type accumulation region 14 of the photodiode 13.
The illustrated pinned photodiode 13 is formed in the p-type substrate 2. It is also possible, for example, to have a p-type substrate base beneath p-wells in an n-type epitaxial layer. The n-type accumulation region 14 and p+ surface accumulation region 5 of the photodiode 13 are spaced between an isolation region 9 and the transfer gate 7. The illustrated conventional pinned photodiode 13 has a p+/n−/p−structure.
The photodiode 13 has two p-type regions 5, 2 having the same potential so that the n− accumulation region 14 is filly depleted at a pinning voltage (Vpin). The photodiode 13 is termed “pinned” because the potential in the photodiode 13 is pinned to a constant value, Vpin, when the photodiode 13 is fully depleted. When the transfer gate 7 is conductive, photo-generated charge is transferred from the n−accumulating region 14 to the floating diffusion region 3.
Additionally, impurity doped source/drain regions 32, having n-type conductivity, are provided on either side of the transistor gates 17, 27, 37 to produce the reset, source follower, and row select transistors, respectively. Conventional processing methods are used to form contacts 33 in an insulating layer to provide an electrical connection 33 to the source/drain regions 32, the floating diffusion region 3, and other wiring to connect to the transistor gates 17, 27, and 37 and to form other connections in the cell 10.
Conventional 4T pixel cells, like the one depicted in FIGS. 1A and 1B, have the advantage over 3T pixel cells of having lower fixed pattern noise. The 4T pixel cells, however, have several drawbacks, which are now discussed generally. First, during the transfer of charges from the photodiode 13 to the floating diffusion region 3, some charges are left behind on the photodiode 13. This incomplete transfer creates lag, and can also lead to saturation of the photodiode 13 due to the presence of excess charge. The traditional 4T design also reduces the fill factor of the cell 10 because the four transistors utilize space that could otherwise be used for a larger photo-sensitive area. As shown in FIG. 1A, the conventional pixel cell 10 has approximately a fifty percent fill factor, as only about half of the cell 10 (i.e., photodiode 13) makes up the photo-sensing area.
There is needed, therefore, a pixel cell having low fixed pattern noise but with a high fill factor, and reduced lag associated with the transferring of photo-charges. There is also a need for a simple method of fabricating the desired cell.